1. Field of the Invention
The present invention relates to an active suspension system for an automotive vehicle. More specifically, the invention relates to an active suspension system which can provide enhanced cornering stability with maintaining riding comfort.
2. Description of the Background Art
The co-pending U.S. patent application Ser. No. 169,201, filed on Mar. 16, 1988, now U.S. Pat. No. 4,865,347, corresponding European Patent Application of which has been published under First Publication No. 02 83 004, discloses an actively controlled suspension system including an anti-roll control loop which is responsive to lateral acceleration to be exerted on a vehicle, to adjust suspension characteristics. The control loop employed suspension system is variable of responsive characteristics to vehicular rolling depending upon the vehicle speed for varying anti-rolling suspension control characteristics. In a range of the vehicle speed, in which the greater loop gain of the control loop, causes significant and unacceptable level of self-induced lateral vibration, the gain is varied to be the smaller value than that in other vehicle speed range. Namely, since the greater gain may affect for causing self-induced lateral vibration in substantially low vehicle speed range, the gain main be adjusted to be smaller while the vehicle speed is lower than a given value.
The co-pending U.S. patent application Ser. No. 176,246, filed on Mar. 31, 1988, now U.S. Pat. No. 4,888,696, corresponding European Patent Application of which has been published under First Publication No. 02 85 153, discloses an actively controlled automotive suspension system employing a factor representative of self-induced attitude change creative factor as a correction factor in attitude regulating suspension control. Suspension characteristics is controlled on the basis of rolling and/or pitching energy representing parameter. The correction factor representative of self-induced attitude change is derived on the basis of vehicular attitude change velocity for canceling the self-induced rolling and/or pitching. By canceling the self-induced rolling and/or pitching creative component from the rolling and/or pitching acceleration factor, higher rolling and/or pitching response characteristics can be provided so as to satisfactory high driving stability and riding comfort.
The co-pending U.S. patent application Ser. No. 172,419, filed on Mar. 24, 1988, now U.S. Pat. No. 4,872,701, corresponding European patent Application of which has been published under First publication No. 02 84 053, discloses a suspension control system for an automotive vehicle which is designed for effectively suppressing pitching motion and whereby regulating vehicular attitude. The suspension control system monitors pitching moment to be exerted on the vehicle for adjusting response characteristics in pitching-suppressive control depending upon the magnitude of the pitching moment to be exerted. The suspension control system adjust the response characteristics of the front and rear suspensions independently of each other according to preset pitching-suppressive control coefficients respectively for the front and rear suspensions, which are set depending upon the suspension characteristics and/or suspension geometory of the vehicle, to which the suspension control system is to be applied.
These prior proposed co-pending applications comprises vertical acceleration sensors for monitoring vertical acceleration exerted on the vehicular body at the orientation where respective suspension systems are provided. Respective vertical acceleration indicative signals are integrated for deriving magnitude of relative displacement between the vehicular body and the corresponding vehicular wheels.
On the other hand, Japanese Patent First (unexamined) publication (Tokkai) Showa 63-227411 discloses another active suspension system, in which speed of relative motion between the vehicle body and the corresponding vehicular wheel for deriving control command for damping vertical vibration. The system further employs a longitudinal acceleration sensor and a lateral acceleration sensor for monitoring magnitudes of pitching and rolling for stabilizing vehicular body attitude.
In such system, it is preferred to provide softer damping characteristics for vertical vibration on a smooth straight road for providing satisfactorily high level riding comfort. On the other hand, in case of cornering on undulated road, softer damping characteristics may lead vehicular cornering stability due to lack of vibration damping force.